1. Field of the Invention
This invention relates to an optical information-recording medium for recording information by utilizing changes in optical properties thereof induced by different thermal histories of a rise and a fall of temperature caused by irradiation of a laser beam thereon, i.e. a phase change optical disk.
2. Description of the Related Art
An optical disk for recording and reproducing information by the use of a laser beam now can be used as a large-capacity portable file memory, and a ROM-type, a direct read after write-type, and a writable-type magneto-optical disks have already been put into practical use. As a type of optical disk which permits overwriting, a phase change optical disk, an exchange coupled magneto-optical disk, and a magnetic field modulation magneto-optical disk are known. Among them, the phase change optical disk is adapted to perform recording and erasing by a change in the optical properties thereof induced by a thermal history of a rise and a fall of temperature caused by irradiation of a laser beam thereon. That is, this type of optical disk is adapted to record information or overwrite new information on old one, by modulating optical energy irradiated thereon between two intensity levels correspondent to two states thereof which can be optically discriminated from each other. Hereinafter, of the two energy (intensity) levels, the higher level will be referred to as a recording power level, while the lower level as an erasing power level. Reproduction of information is performed normally by detecting changes in reflectivity or transmittance between the two states.
To improve the recording density of the optical disks, a mark edge recording method in which information is imparted to both ends of each recording mark is now under study or development. A recording medium used in the phase change optical disk is generally higher in absorptivity when it is in the amorphous state than when it is in the crystalline state, and therefore, in performing overwriting by the mark edge recording method, the length or position of each recording mark recorded varies depending on whether the recording mark is recorded on the amorphous state or on the crystalline state, and this results in degraded overwriting characteristics, such as increased recording jitter, and a lower erasability resulting from modulation of an overwriting signal, which occurs in dependence on the existing or recorded information. Further, when taking into consideration the fact that more latent heat is consumed in fusion and the thermal conductivity is larger in the crystalline state than in the amorphous state, the layer structure of a phase change optical disk should be designed such that absorptivity in the crystalline state becomes higher than one in the amorphous state.
As means for providing such a recording medium, an invention proposed by Japanese Unexamined Patent Publication (Kokai) No. 1-149238 is known. In this invention, absorptivity in the crystalline state is increased by reducing the thickness of a reflective layer formed of a metal to make it transparent, thereby reducing the reflectivity and absorptivity of the reflective layer. However, when the thickness of the metal reflective layer is set to a reduced value, optical characteristics of the resulting medium largely depends on the layer thickness, and hence there arises a problem of a strict or close manufacturing tolerance of thickness of the reflective layer. FIG. 1 shows the relationship between the thickness of a single Au layer formed on a glass substrate and reflectivity of the layer exhibited in reflecting light irradiated on the substrate. The invention disclosed in Japanese Unexamined Patent Publication (Kokai) No. 1-149238 obtains desired effects by reducing the thickness of the Au layer as the reflective layer to 20 nm, thereby lowering the reflectivity of the reflective layer. However, as can be understood from FIG. 1, the resulting reflectivity largely depends on the layer thickness, when the layer thickness is small, for example 20 nm or less. Therefore, if the layer thickness is largely deviated from a specified value, this causes a large undesirable change in the reflectivity of the reflective layer, resulting in a large amount of deviation from the desired optical properties of the recording medium. In short, the manufacturing tolerance of the thickness of the reflective layer is so strict or close. Further, when the thickness of the reflective layer is small, the cooling rate of the medium becomes low, and hence thermal load thereon becomes large, which can cause a problem of degraded characteristics occurring with repetition of recording and erasing.
To make the absorptivity in the crystalline state higher than that in the amorphous state, there is proposed another method which employs optical phase encoding (i.e. optical phase difference reproduction). In the case of the phase change optical disk, in general, the difference in reflectivity between the amorphous state and the crystalline state is detected for reproduction of information, and to increase the signal strength, it is required to increase the difference in reflectivity between the amorphous state and the crystalline state. Normally, the recording medium is more often used after being crystallized. Therefore, the reflectivity in the crystalline state is set to a higher value, and the absorptivity of the crystalline state is set to a lower value when the disk is arranged to include a reflective layer. However, if the method of optical phase encoding is used, in which the optical phase difference between light reflected from the amorphous state and light reflected from the crystalline state is utilized, it is not necessarily required to increase the difference in reflectivity to enhance the signal strength, which facilitates increasing the absorptivity in the crystalline state. Examples of such phase difference reproduction are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-73537, Japanese Unexamined Patent Publication (Kokai) No. 2-113451, Japanese Unexamined Patent Publication (Kokai) No. 3-41638, etc. These inventions, however, are constructed such that there is little difference in reflectivity between the amorphous state and the crystalline state, with a large phase difference between reflected lights, and hence, optical characteristics of the recording medium largely depend on the layer thickness, which results in a small manufacturing tolerance of layers. Further, according to the layer arrangement disclosed in preferred embodiments of the known publications, reflectivity is set to a value as low as approximately 8% or lower, which results in a problem of unstable servo-mechanism.